ACI CODE fpy vs fpu Chapter 18.7

[LakeNormanTiger]

Friends,

I am a relatively new engineer, assigned to train some new engineers prestress design (Get them up to speed on basics). Currently we are reviewing flexural design and using the strain compatiability process.

We are at the point of choosing a value for "gamma,p" Factor for prestressing tendon. In ACI-318 18.7.2 you are given different values with respect to the fpy/fpu ratio. I was told to use a ratio of 0.85 for fpy/fpu and the subsequent value for gamma,p is 0.4. One of my resourcefull new hires went throught the PCA Notes on ACI-318(pg. 24-24) and they use a ratio = 0.9 for there ratio of fpy/fpu which has a value of 0.28 for gamma,p. At this point I am at a loss.

My understanding is that the fpu term is the ultimate value at the top of the stress strain curve. I have always called it yield strength. Because once you pass this point the material looses its elastic ability. fpy, to me, is a built in safety factor. This ratio keeps you from going near the fpu value.

So here I guess is my question. Why are there two different values for fpy/fpu (0.85 and 0.9) that the code gives values for gamma,p? Why not just one? I know there are two types of strands that are used (low lax and stress relieved), but I thought we are just looking at yeild strengh. Why should this ratio be different between the two types of strands if they have the same yield strength?

Also, am I missing the bigger picture? What am I overlooking so that A)I know a little more, and B) I can teach these new engineers correctly.

Thanks in advance for any help you can give.

LakeNormanTiger

 

[civilperson]

The stress/strain curve of strand does not have a defined yield point. The fraction of available strength used can be adjusted as a time variable, the stretching and then jump back of the strands into the jaws or block can be 3/8 to 1/2". A value of 0.85 used after the jaws have grabbed the strand is close to 0.9 of maximum stretch for before release, (for casting beds of medium to large lengths).

 

[Lion06]

use gammap = 0.28 for low-lax strands and 0.4 for stress-relieved strands.

 

[shin25]

fpu = ultimate strength of the prestressing strand. For low relaxation strand, this is usually 270 ksi. All available strands now a days are of this kind.

fpy= yield strength of the strand, which is .9*fpu for low relaxation strand.

ASTM reference for this material is ASTM A416.

 

[shin25]

"My understanding is that the fpu term is the ultimate value at the top of the stress strain curve. I have always called it yield strength. Because once you pass this point the material looses its elastic ability. fpy, to me, is a built in safety factor. This ratio keeps you from going near the fpu value."

-you probably need a little more basic on this as well!!

 

[rapt]

LakeNormanTiger,

I will not comment oin your first sentence!

If you are doing the calculations by strain compatibility you do not use the formula in 18.7.2. The fps formula is used as an alternative to doing strain compatibility calculations. Make up your mind!

fpu is the nominal breaking load of the strand.
fpy is the nominal yield strength of strand (as mentioned by someone earlier there is no definite yield point for strand).You would need to look at ASTM A 421 for the definition of yield for prestressing strand.

 

[LakeNormanTiger]

All,

Frist thanks to all that provided help.

Second, I am sorry for labeling the method of flexural design "Strain Compatibility". The method is a strength design process. I have always associated the fps equation that is located in section 18.7.2 as "strain compatibility" since it is trying to give a more precise answer like strain compatibility method would do. Sorry about that, it is a bad habbit that I picked up and will try not to do it again.

Third, for those of you that have directed me to the ASTM A416 thank you. Currently, I do not have one of those in my office. I still do not understand why there are different ratios for the different types of strands, but hopefully that resource will tell me why when I get a hold of it.

And last, I would like to thank the folks who took the time to take a jab at me. I thought I tried to qualify myself as a person who did not know that much, and obivously did a good job of it by looking at two of the responses. I hope it made your day better, making someone feel stupid. I only hope that when you have a question about a topic that you have little or no expereince with, that the people you reach out to for help dont make you feel like an idiot. It is becoming clearer everyday why structural engineers have the reputation that they do.

Thanks for the help
LakeNormanTiger

 

[rapt]

LakeNormanTiger,

I was not having a go at YOU.

My comment would have been about your superiors who have assigned a "relatively new engineer" (assumption: relatively inexperienced) to "to train some new engineers prestress design" (assumption: no knowledge).

I hope you are teaching them from a good text book on PT and not the ACI code illogic for flat slabs.